The conventional auto exhaust gas purifiers include a manifold reactor, a catalyst converter and so on. For the purpose of preserving the internal heat of these devices, while reducing their exhaust gas emission and preventing the heat in these devices from being transferred to the other auto parts with adverse effect, a heat insulating material which is durable at high temperatures is required. The heat resistance of the foamed sodium silicate ceramics heretofore used as heat insulating materials can be improved by adding aluminum hydroxide, but 500.degree.-600.degree. C is said to be the upper limit to which the ceramics thus improved can resist heat.
At the same time, ceramic fibers are often used for insulation of manifold reactors, but they cause various problems in that the fibers tend to fly out into the exhaust gas through gaps in welded joints which have been broken under repeated application of thermal stresses due to temperature variations, or to clog the bypass valve of the catalyst converter or the valve of the exhaust gas recirculator (EGR).